Retinal rod outer segment which is the photo receptor unit of the neuro retina contains extraordinarily high enzyme activities for the synthesis and degradation of cyclic GMP. Moreover, it contains very high protein kinase activity which specifically phosphorylates opsin. The role of these enzymes in vision is not clearly understood. The main objective of this study is to examine the effect of light exposure of the rod outer segments on the activities of these enzyme systems and to indicate the possible role of cyclic GMP in vision.